staging: comedi: gsc_hpdi: remove board(dev) function

The `board(dev)` function just casts `dev->board_ptr` to a pointer to
the private data type `struct hpdi_private` and returns it.  It's only
called from one function: `hpdi_auto_attach()`.

Remove `board()` and use a local variable to point to the `struct
hpdi_private` data structure.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>